1. Field of the Invention
The present invention relates to vibratory seismic energy sources used for seismic prospecting. More particularly, it relates to a system for controlling the vibratory force imparted to the ground.
2. Description of the Prior Art
In seismic prospecting, it has become common to use, as a seismic wave source, an electrohydraulically controlled vibratory source, more simply referred to as a vibrator.
Typically, a vibrator comprises a double-ended piston rigidly affixed to a coaxial piston rod. The piston is located in reciprocating relationship in a cylinder formed within a heavy reaction mass. Means are included for alternately introducing hydraulic fluid under high pressure to opposite ends of the cylinder, thereby imparting a recirocating motion to the piston relative to the reaction mass. The piston rod extending from the reaction mass is rigidly coupled to a ground coupling plate (hereinafter baseplate) which is maintained in intimate contact with the gound. The inertia of the reaction mass tends to resist displacement of the reaction mass relative to the earth. The motion of the piston is coupled through the piston rod and baseplate to impart vibratory seismic energy in the earth.
Typically, a vibrator is vehicle mounted. Upon arrival at a selected location, the baseplate is lowered into contact with the earth's surface, and to prevent decoupling of the baseplate from the ground during operation, a portion of the vehicle's weight is applied to the baseplate. The weight of the vehicle is frequently applied to the baseplate through one or more spring members so that a static bias force is imposed on the baseplate while the dynamic forces of the baseplate are decoupled from the vehicle.
Because of variation in the earth's impedance at different locations, and variations in impedance with frequency at a given location, the maximum vibrator baseplate motion that can be generated while keeping the baseplate in contact with the ground may vary considerably depending on the soil conditions and the frequency. Normally, drive levels are now set by trial and error; that is, if the baseplate is observed to decouple from the ground, the drive level is decreased; if it doesn't, the drive level might be increased.
Typically, the vibrator is driven with a "swept sine" wave in a manner well known to the industry. With this form of signal the phase of the input signal is controlled by a feedback control loop, but feedback is not used to control the vibratory amplitude.
It has been proposed that the vibratory amplitude be controlled by controlling the instantaneous motion of the baseplate. Castanet, et al, in U.S. Pat. No. 3,208,550 proposes to generate a signal proportional to force generated by the baseplate and utilize this signal in a feedback network to vary the instantaneous drive level to the vibrator. Mifsud, in U.S. Pat. No. 4,049,077, proposes to generate a feedback signal equal to the instantaneous amplitude of either coupling plate velocity or displacement and utilize one of these feedback signals to control the instantaneous motion of the coupling plate.
The system proposed by Castanet, et al, may be difficult to implement because variations in ground impedance at different locations and variations in ground impedance with frequency tend to make such a control loop unstable. When utilizing the control loop of Mifsud, it is necessary to determine in advance, for a particular location, the maximum velocity or displacement amplitude that can be generated over the frequency range of interest without decoupling of the baseplate from the ground.